Currently deployed across a broad utility spectrum in battery engineering is the lithium ion technology. Notable features of this technology include its high energy density and its extremely low inherent self-discharge. Lithium ion cells possess at least one positive electrode (cathode) and a negative electrode (anode), which are capable of reversible insertion—known as intercalation—or removal—known as deintercalation—of lithium ions.
Lithium ion cells are generally packed in composite aluminum foils or possess a hard metallic housing. On account of the pliant packaging, battery cells are also referred to as pouches or soft packs. Housings for cells with a hard, purely metallic housing are also referred to as hardcases.
For intercalation of lithium ions and/or deintercalation of lithium ions to take place at all, the presence is required of a lithium ion conductive salt. For the majority of contemporary lithium ion cells, both in the consumer sector (mobile telephone, MP3 player, etc.) and in the automotive sector—HEV (Hybrid Electric Vehicle), PHEV (Plug-in Hybrid Electric Vehicle), EV (Electric Vehicle)—the lithium conductive salt used with preference is lithium hexafluorophosphate (LIPF6). The Li+ ions migrate back and forth through a porous separator between the electrodes during the charging operation and during the discharge operation of the lithium ion cell.
On account of the high specific energy and volumetric energy density of the lithium ion cells, cell materials employed in these cells are required to have a high level of intrinsic safety and also a safe cell design. This is ensured by means including a separator which separates the negative electrode from the positive electrode and itself constitutes an electrical insulator.
In the lithium ion technology in accordance with the prior art it is possible in lithium ion cells to use porous polyolefin separators. Polyolefin separators designed with this kind of porosity may be polyethylene (PE) or polypropylene (PP) based. Specifically with polyethylene, and also, moreover, with polypropylene, temperatures in the region of the softening point of the polymer are accompanied by contraction around the sides of the polyolefin separator. The term “shrinking” is also used. Other separators used, made of plastic which is high-melting, are known from U.S. Pat. No. 7,112,389 B1 and are more stable both thermally and mechanically than polyolefin-based separators. In high-capacity automotive cells with a capacity of around 20 to 90 Ah, even the polyimide-based separators from U.S. Pat. No. 7,112,389 B1, which have improved properties in contrast to polyolefin separators, often lack sufficient intrinsic safety, particularly under mechanically and thermally induced loads and also under electrical stress such as overcharging, for example.
An alternative route is taken in the solution according to DE 10 2009 002689 A1. This solution discloses the production and use of ceramic composite materials based on polymer support sheet. Here, a composite is used which is composed of a porous ceramic layer and a porous plastics film, in order to constitute a separator. As a plastics membrane of porous design, this solution uses a perforated sheet having a regular arrangement of holes. A configuration of this kind with an open porosity, however, carries the disadvantage that during the charging operation of the cell, particularly in the case of a high charge state in a constant voltage recharge, it is possible for lithium dendrites to form, which might lead to internal cell shortcircuits and initiate safety-critical events.